I hit a non-deterministic SPI bus crash on the ESP32-S3 that took days to diagnose. Here’s what caused it and the protocol I built to prevent it. Long post. Worth it if you’re building anything concurrent on the S3. \*\*The symptom:\*\* Guru Meditation errors during normal operation. The crash address changed every single time. Sometimes it pointed at the display driver. Sometimes the SD card library. Sometimes an audio buffer. Looked like three different bugs. It was one bug. \*\*The hardware:\*\* LilyGO T-Deck Plus. ESP32-S3 dual-core, 8MB PSRAM, MicroSD, SX1262 LoRa radio, GPS, WiFi, BLE. I was building a general-purpose OS — the Ghost Engine runs continuous background wardriving on Core 0 while Core 1 handles the UI and applications. 47 apps. Always-on background intelligence collection. \*\*The actual cause:\*\* The MicroSD card and LoRa radio share the SPI bus. Separate chip-select lines but shared MOSI/MISO/CLK.…